Communication apparatus, communication method, computer program, and communication system

ABSTRACT

Even when the lengths of data items to be transmitted to users are not the same, the frames multiplexed at the same time finally have the same frame length and are transmitted. 
     Even when the lengths of frames for the users are not the same at the time when a transmission request is received from a higher layer, a communication apparatus reconfigures at least two of the frames having short lengths into a frame having a long length through Aggregation so that the frames finally have the same frame length and transmits the frames at the same time in a multiplexed manner. On the transmitter side, the transmission power used per destination communication station can be increased due to a decrease in the total number of multiplexed frames. On the receiver side, an unstable AGC operation can be prevented.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application of U.S. patentapplication Ser. No. 15/791,502, filed Oct. 24, 2017 which is acontinuation application of U.S. patent application Ser. No. 15/471,068,filed Mar. 28, 2017, now U.S. Pat. No. 9,847,826, which is acontinuation application of U.S. patent application Ser. No. 14/482,064,filed Sep. 10, 2014, now U.S. Pat. No. 9,712,336, which is acontinuation application of U.S. patent application Ser. No. 13/266,142,filed Oct. 25, 2011, now U.S. Pat. No. 8,929,286, which is a NationalStage Entry of PCT/JP2010/056920, filed Apr. 19, 2010, and claims thebenefit of priority from prior Japanese Patent Application JP2009-113869, filed May 8, 2009, the entire content of which is herebyincorporated by reference.

TECHNICAL FIELD

The present invention relates to a communication apparatus, acommunication method, a computer program, and a communication systemthat employ Space Division Multiple Access (SDMA) in which a pluralityof users share wireless resources on a spatial axis and, in particular,to a communication apparatus, a communication method, a computerprogram, and a communication system that multiplex frames having avariable length frame format and destined for a plurality of users andtransmit the frames.

BACKGROUND ART

Wireless communication has been developed and used as a communicationtechnique for eliminating a wiring operation required in existing wiredcommunication and further realizing mobile communication. For example,an example of the standard for a wireless LAN (Local Area Network) isIEEE (The Institute of Electrical and Electronics Engineers) 802.11.IEEE 802.11a/g has already been widespread.

The IEEE 802.11a/g standard supports a modulation method that realizes amaximum communication speed (the physical layer data rate) of 54 Mbpsusing Orthogonal Frequency Division Multiplexing (OFDM) in the 2.4 GHzband or the 5 GHz band. In addition, in the IEEE 802.11n standard, whichis an extension of the IEEE 802.11a/g standard, a higher bit rate isrealized by employing the MIMO (Multi-Input Multi-Output) communicationscheme. The MIMO communication scheme is a communication scheme (anexisting communication scheme) that realizes a spatially multiplexedstream using a transmitter and a receiver each including a plurality ofantennas. IEEE 802.11n can provide a high throughput that is higher than100 Mbps. However, with an increase in the amount of information oftransmitted content, a higher bit rate is required.

For example, by increasing the number of antennas of the MIMOcommunication devices and increasing the number of spatially multiplexedstreams, the throughput of pier-to-pier communication can be increasedwhile maintaining downward compatibility. However, in the future, thethroughput of communication among a plurality of users needs to beincreased in addition to increasing the throughput per user incommunication.

The working group of IEEE 802.11ac attempts to establish a wireless LANstandard that uses a frequency band lower than or equal to 6 GHz andthat realizes a data transmission speed that is higher than 1 Gbps. Inorder to realize such a wireless LAN standard, a space division multipleaccess scheme in which a plurality of users share wireless resource onthe spatial axis, such as multi-user MIMO (MU-MIMO) or SDMA (SpaceDivision Multiple Access), is a promising scheme.

Currently, space division multiple access is developed as one of basetechnologies for a next-generation cell phone system based on TimeDivision Multiple Access (TDMA), such as PHS (Personal HandyphoneSystem) or LTE (Long Term Evolution). In addition, in the wireless LANtechnical field, one-to-many communication garners much attention, asdescribed above. However, few applications are available in this field.One of the reasons for that is that it is difficult to efficientlymultiplex a plurality of users in packet communication.

Note that a communication system has been developed by using an RTSpacket, a CTS packet, and an ACK packet that have a packet format havingdownward compatibility with IEEE 802.11 and combining the following twotechniques: carrier sense of the existing IEEE 802.11 standard and spacedivision multiple access using an adaptive array antenna (refer to, forexample, PTL 1).

When the space division multiple access scheme is applied to a wirelessLAN, a variable length frame may be multiplexed on the same time axis.At that time, if the lengths of data items transmitted to all of theplurality of users are the same, no problem arises. However, if thelengths of all frames to be multiplexed are not the same due to adifference among the lengths of transmitted data, the level of framemultiplexing during a transmission interval is decreased or increasedand, therefore, the total transmission power is abruptly changed. Ifframes having different lengths are directly multiplexed andtransmitted, the received power is abruptly changed on the receiver sidedue to an increase or a decrease in the level of frame multiplexing.Thus, an unstable operation occurs in terms of auto gain control (AGC).In this way, a variety of problems may arise (e.g., the powerdistribution in a frame in terms of an RCPI (Received Channel PowerIndicator) defined in IEEE 802.11 varies). Accordingly, even when thelengths of data items transmitted to the users are not the same, theframes multiplexed on the same time axis need to be finally transmittedwhile keeping the same frame length.

For example, in the systems having a fixed frame format (such as anexisting cellar system), a frame, for example, can be padded byinserting diversity data (refer to, for example, PTL 2), schedulingassigned times (refer to, for example, PTL 3), using a variable datarate (refer to, for example, PTL 4 or 5), or using a variable channelconfiguration (refer to, for example, PTL 6). However, since theconfiguration of such a system radically differs from the configurationof a system using a variable length frame format, such as a wireless LANsystem, it is difficult to apply such existing technologies to thesystem using a variable length frame format.

CITATION LIST Patent Literature

PTL 1: Japanese Unexamined Patent Application Publication No.2004-328570

PTL 2: Japanese Unexamined Patent Application Publication No.2001-148646

PTL 3: Japanese Unexamined Patent Application Publication No.2009-506679

PTL 4: Japanese Unexamined Patent Application Publication No.2008-236065

PTL 5: Japanese Patent No. 2855172

PTL 6: Japanese Unexamined Patent Application Publication No. 2007-89113

DISCLOSURE OF INVENTION Technical Problem

It is an object of the present invention to provide excellentcommunication apparatus, communication method, computer program, andcommunication system capable of appropriately performing a communicationoperation using the space division multiple access in which a pluralityof users share wireless resources on the spatial axis. It is anotherobject of the present invention to provide excellent communicationapparatus, communication method, computer program, and communicationsystem capable of multiplexing frames having a variable length frameformat and destined for a plurality of users and optimally transmittingthe frames.

It is still another object of the present invention to provide excellentcommunication apparatus, communication method, computer program, andcommunication system capable of suitably transmitting frames having avariable-length frame format in a multiplexed manner while avoiding anabrupt change in the total transmission power on the transmitter sideeven when the lengths of transmission data items destined for aplurality of users are not the same.

Solution to Problem

In order to solve the above-described problems, the present invention isprovided. According to the invention described in Claim 1, acommunication apparatus includes a frame generating unit configured togenerate a plurality of frames to be transmitted at the same time, aframe integration processing unit configured to integrate at least twoof the frames generated by the frame generating unit into a singleframe, and a communication unit configured to transmit the frames at thesame time in a multiplexed manner.

According to the invention described in Claim 2, the communicationapparatus according to Claim 1 further includes a frame length controlunit configured to adjust the lengths of the plurality of framesmultiplexed at the same time so that the lengths are finally made thesame.

According to the invention described in Claim 3, the frame lengthcontrol unit of the communication apparatus according to Claim 2integrates at least two frames having short lengths into a single framehaving a long length.

According to the invention described in Claim 4, the communication unitof the communication apparatus according to Claim 3 includes a pluralityof antenna elements capable of functioning as an adaptive array antennaby assigning weights to the antenna elements and can transmit theplurality of frames at the same time in a multiplexed manner. Inaddition, if the lengths of the frames are not the same, the framelength control unit extracts a combination of communication apparatusesappropriate for combining the antenna weights from among thecommunication apparatuses defined as destinations of the frames andadjusts the lengths of the frames by integrating the frames destined forthe communication apparatuses in the combination using the frameintegration processing unit.

According to the invention described in Claim 5, if the frame lengthcontrol unit of the communication apparatus according to Claim 4 isfinally unable to make the frame lengths the same through frameintegration, the frame length control unit appropriately performspadding on the frame having a short length so that the frames finallyhave the same length.

According to the invention described in Claim 6, a communication methodincludes a frame generating step of generating a plurality of frames tobe transmitted at the same time, a frame integration processing step ofintegrating at least two of the frames generated in the frame generatingstep into a single frame, and a communication step of transmitting theplurality of frames at the same time in a multiplexed manner.

According to the invention described in Claim 7, a computer programwritten in a computer-readable format so that a process for acommunication apparatus to transmit a frame is executed by a computer isprovided. The program includes code causing the computer to functions asa frame generating unit configured to generate a plurality of frames tobe transmitted at the same time, a frame integration processing unitconfigured to integrate at least two of the frames generated by theframe generating unit into a single frame, and a communication unitconfigured to transmit the plurality of frames at the same time in amultiplexed manner.

The computer program according to Claim 7 defines a computer programwritten into a computer-readable format so that a predetermined processis performed by a computer. That is, by installing the computer programaccording to Claim 7 of the invention in a computer, cooperativeprocesses are performed in the computer. Thus, the operations andadvantages that are the same as those of the communication apparatusaccording to Claim 1 of the invention can be obtained.

In addition, as used herein, the term “system” refers to a logicalcombination of a plurality of devices (or functional modules that eachrealizes a particular function); the plurality of devices or functionalmodules are not necessarily included in one body. According to theinvention described in Claim 8, a communication system includes a firstcommunication apparatus configured to integrate at least two framesamong a plurality of frames to be transmitted at the same time into asingle frame and transmit the frames at the same time in a multiplexedmanner and a plurality of second communication apparatuses configured toreceive the frames multiplexed at the same time.

Advantageous Effects of Invention

According to the present invention, excellent communication apparatus,communication method, computer program, and communication system capableof appropriately performing a communication operation using the spacedivision multiple access in which a plurality of users share wirelessresources on the spatial axis can be provided.

Furthermore, according to the present invention, excellent communicationapparatus, communication method, computer program, and communicationsystem capable of suitably transmitting frames having a variable-lengthframe format in a multiplexed manner while avoiding an abrupt change inthe total transmission power on the transmitter side even when thelengths of transmission data items destined for a plurality of users arenot the same can be provided.

According to the invention described in Claims 1 and Claims 6 to 8, whena communication apparatus multiplexes a plurality of frames at the sametime and transmits the frames, a process for integrating at least twoframes among the plurality of frames into a single frame, that is,Aggregation, can be performed as needed. By employing a frameintegration process in the space division multiple access scheme, thetotal number of multiplexed frames can be reduced. Accordingly, thetransmission power per communication station defined as a destination isincreased on the transmission side, and an increase in the communicationquality can be expected.

According to the invention described in Claim 2, even when the lengthsof frames destined for the users are not the same when a transmissionrequest is delivered from a higher layer, the frames multiplexed at thesame time can be finally made the same and can be transmitted.Accordingly, frames having a variable-length frame format can bemultiplexed and appropriately transmitted while avoiding an abruptchange in the total transmission power on the transmission side. As aresult, on the reception side that receives the multiplexed frames, anunstable AGC operation due to an abrupt change in the received power canbe prevented.

According to the invention described in Claim 3, by integrating at leasttwo frames having short lengths into a frame having a long length, theframes multiplexed at the same time can finally have the same length andcan be transmitted. Accordingly, frames having a variable-length frameformat can be appropriately multiplexed and transmitted while avoidingan abrupt change in the total transmission power on the transmissionside. As a result, on the reception side that receives the multiplexedframes, an unstable AGC operation due to an abrupt change in thereceived power can be prevented.

According to the invention described in Claim 4, in a communicationapparatus that performs a space division multiple access using anadaptive array antenna, by extracting, from among communicationapparatuses defined as destinations of a plurality of frames transmittedin a multiplexed manner, a combination of the communication apparatusesappropriate for combining the antenna weights, the frame integrationprocessing unit can integrate the frames destined for the communicationapparatuses in the combination and adjust the frame lengths. That is, bycombining Aggregation of frames with the space division multiple accessscheme, overhead can be reduced and the throughput of one-to-manycommunication can be increased at the same time.

According to the invention described in Claim 5, even when the lengthsof the frames cannot be finally made the same through integration of theframes, the frame lengths can be finally made the same through a paddingprocess performed on the frames having short lengths as needed.Therefore, the transmission power per frame can be increased by reducingthe total number of multiplexed frames through Aggregation of theframes. In addition, by making the frame lengths the same, an unstableAGC operation on the receiver side can be prevented.

Further features and advantages of the present invention will becomeapparent from the following detailed description of exemplaryembodiments of the present invention with reference to the attacheddrawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic illustration of the configuration of acommunication system according to an embodiment of the presentinvention.

FIG. 2 illustrates an example of the configuration of a communicationapparatus capable of multiplexing a plurality of users through spacedivision multiple access.

FIG. 3 illustrates an example of the configuration of a communicationapparatus that complies with an existing standard, such as IEEE 802.11a,and that does not use the space division multiple access.

FIG. 4 illustrates an example of a processing sequence of thecommunication system illustrated in FIG. 1 in which the communicationstation STA0 operating as an access point serves as a data source,communication stations STA1 to STA3 operating as client devices serve asdata destinations, and STA0 multiplexes frames to be transmitted to thecommunication stations STA1 to STA3 on a spatial axis and transmits theframes at the same time.

FIG. 5 illustrates frames A and B that have different lengths and thatare multiplexed on the same time axis.

FIG. 6 illustrates the case in which when a plurality of frames havingdifferent lengths are multiplexed at the same time, the frames aresubjected to a padding process.

FIG. 7 illustrates the case in which at least two frames having shortlengths are integrated into a single frame by Aggregation so that thelengths of frames are adjusted with respect to the length of a framehaving a long length.

FIG. 8 is a flowchart of a processing sequence executed by thecommunication apparatus illustrated in FIG. 2 when, in the communicationsequence illustrated in FIG. 7, the communication apparatus functions asan access point (STA0) and transmits frames destined for a plurality ofcommunication stations at the same time in a multiplexed manner.

FIG. 9 is a flowchart of a processing sequence executed by thecommunication apparatus illustrated in FIG. 2 when, in the communicationsequence illustrated in FIG. 7, the communication apparatus functions asany one of the client devices (STA1 to STA3) and receives framestransmitted from an access point at the same time in a multiplexedmanner.

DESCRIPTION OF EMBODIMENTS

Embodiments of the present invention are described in detail below withreference to the accompanying drawings.

FIG. 1 is a schematic illustration of the configuration of acommunication system according to an embodiment of the presentinvention. The communication system illustrated in the drawing includesa communication station STA0 operating as an access point (AP) and aplurality of communication stations STA1, STA2, and STA3 each operatingas a client device (MT).

The communication area of each of the communication stations STA1, STA2,and STA3 includes the communication station STA0, and each of thecommunication stations STA1, STA2, and STA3 can directly communicatewith the communication station STA0 (that is, the communication stationsSTA1, STA2, and STA3 are controlled by the communication station STA0serving as an access point and form a BSS (Basic Service Set)). However,each of the communication stations STA1, STA2, and STA3 serving asclient devices need not be located within the communication areas of theother communication stations. Hereinafter, direct communication amongthe client devices is not discussed.

Here, the communication station STA0 serving as an access point isformed from a communication apparatus that includes a plurality ofantennas and performs the space division multiple access using anadaptive array antenna. The communication station STA0 assigns wirelessresources on the spatial axis to the plurality of users and multiplexesframe communication. That is, the communication station STA0 is acommunication apparatus that complies with a new standard such as IEEE802.11ac. The communication station STA0 multiplexes two or more framesdestined for different communication stations on the same time axis andseparates a frame destined for the communication station STA0 andmultiplexed for two or more communication stations into frames for thesource communication stations. In this way, the communication stationSTA0 performs one-to-many frame communication. The communication stationSTA0 can increase the number of client devices capable of performing thespace division multiple access by increasing the number of antennasthereof. Of course, in addition to performing one-to-many framecommunication with the communication stations STA1, STA2, and STA3 usingthe space division multiple access, the communication station STA0 mayperform pier-to-pier communication with each of the communicationstations STA1, STA2, and STA3.

In contrast, each of the communication stations STA1, STA2, and STA3serving as a client device is formed from a communication apparatus thatincludes a plurality of antennas and performs the space divisionmultiple access using an adaptive array antenna. However, userseparation is performed only when a receiving operation is performed.When a transmitting operation is performed, user separation, that is,multiplexing of transmission frames is not performed. Accordingly, thenumber of antennas may be less than that of the access points. Note thatat least some of the client devices under the control of thecommunication station STA0 serving as an access point may becommunication apparatuses that comply with the existing standard, suchas IEEE 802.11a. That is, the communication system illustrated in FIG. 1is in an environment in which communication apparatuses that comply withthe new standard coexist with communication apparatuses that comply withan existing standard.

FIG. 2 illustrates an example of the configuration of a communicationapparatus capable of multiplexing a plurality of users through spacedivision multiple access. Among the communication stations in thecommunication system illustrated in FIG. 1, the communication stationSTA0 serving as an access point and any one of the communicationstations STA1 to STA3 that serves as a client device and that employsthe space division multiple access scheme have the configurationillustrated in FIG. 2 and perform a communication operation inaccordance with the new standard.

The communication apparatus illustrated in the drawing includes Ntransmitting and receiving branches 20-1, 20-2, . . . , and 20-N havingantenna elements 21-1, 21-2, . . . , and 21-N, respectively, and a dataprocessing unit 25 connected to the transmitting and receiving branches20-1, 20-2, . . . , and 20-N (where N is an integer greater than orequal to 2). The data processing unit 25 processes data to betransmitted and received data. When appropriate weights for an adaptivearray antenna are assigned to the plurality of antenna elements 21-1,21-2, . . . , and 21-N, the antenna elements 21-1, 21-2, . . . , and21-N can function as an adaptive array antenna. The communicationstation STA0 serving as the access point performs the space divisionmultiple access using the adaptive array antenna. By increasing thenumber of antenna elements included therein, the communication stationSTA0 can increase the number of client devices included in the multipleaccess.

In the transmitting and receiving branches 20-1, 20-2, . . . , and 20-N,the antenna elements 21-1, 21-2, . . . , and 21-N are connected totransmission processing units 23-1, 23-2, . . . , and 23-N and receptionprocessing units 24-1, 24-2, . . . , and 24-N via duplexers 22-1, 22-2,. . . , and 22-N, respectively.

The data processing unit 25 generates data to be transmitted in responseto a transmission request received from a higher-layer application and,thereafter, sorts the data into the transmitting and receiving branches20-1, 20-2, . . . , and 20-N. In addition, in the communication stationSTA0 serving as an access point, the data processing unit 25 generates aplurality of data items to be transmitted to the plurality of users,that is, data item to be transmitted to the communication stations STA1,STA2, and STA3 in response to a transmission request received from ahigher-layer application and, thereafter, multiplies the data item by atransmission weight of the adaptive array antenna for each of thetransmitting and receiving branches. In this way, the data items arespatially separated and sorted into the transmitting and receivingbranches 20-1, 20-2, . . . , and 20-N. Note that as used herein, theterm “spatial separation” in transmission refers to only user separationin which frames transmitted at the same time are spatially separated forthe users.

Each of the transmission processing units 23-1, 23-2, . . . , and 23-Nperforms predetermined signal processing, such as encoding andmodulation, on a digital baseband transmission signal supplied from thedata processing unit 25 and, thereafter, performs D/A conversion on thedigital baseband transmission signal. Subsequently, each of thetransmission processing units 23-1, 23-2, . . . , and 23-N upconvertsthe digital baseband transmission signal into an RF (Radio Frequency)signal. Thus, the power of the signal is amplified. Subsequently, suchtransmission RF signals are supplied to the antenna elements 21-1, 21-2,. . . , and 21-N via the duplexers 22-1, 22-2, . . . , and 22-N. Thus,the transmission RF signals are emitted into the air.

In contrast, upon receiving the RF reception signals from the antennaelements 21-1, 21-2, . . . , and 21-N via the duplexers 22-1, 22-2, . .. , and 22-N, the reception processing units 24-1, 24-2, . . . , and24-N low-noise amplify the RF reception signals. Thereafter, thereception processing units 24-1, 24-2, . . . , and 24-N down-convert theRF reception signals into analog baseband signals and D/A-convert theanalog baseband signals. Furthermore, the reception processing units24-1, 24-2, . . . , and 24-N perform predetermined signal processing,such as decoding and demodulation, on the analog baseband signals.

The data processing unit 25 multiplies the digital reception signalinput from each of the reception processing units 24-1, 24-2, . . . ,and 24-N by a reception weight of the adaptive array antenna andperforms spatial separation. In this way, the transmission data itemstransmitted from the users, that is, the communication stations STA1,STA2, and STA3, are reconstructed. Thereafter, the data processing unit25 delivers the reconstructed transmission data items to a higher-layerapplication. Note that as used herein, the term “spatial separation” inreception refers to user separation in which frames transmitted at thesame time are spatially separated for the users and channel separationin which a spatially multiplexed MIMO channel is separated into theoriginal multiple streams.

At that time, in order to cause the antenna elements 21-1, 21-2, . . . ,and 21-N to function as an adaptive array antenna, the data processingunit 25 controls the transmission processing units 23-1, 23-2, . . . ,and 23-N and the reception processing units 24-1, 24-2, . . . , and 24-Nso that the transmission data items sorted into the transmitting andreceiving branches 20-1, 20-2, . . . , and 20-N are multiplied by thetransmission weights of the adaptive array antenna and the receptiondata items received from the transmitting and receiving branches 20-1,20-2, . . . , and 20-N are multiplied by the reception weights of theadaptive array antenna. In addition, before performing the spacedivision multiple access with the communication stations STA1, STA2, andSTA3, the data processing unit 25 learns the weights of the adaptivearray antenna. For example, the data processing unit 25 can learn theweights of the adaptive array antenna using training signals (describedin more detail below) received from the communication partners STA1,STA2, and STA3 in a known sequence and a predetermined adaptivealgorithm, such as an RLS (Recursive Least Square) algorithm.

For example, the data processing unit 25 performs a process in each ofthe layers of a communication protocol of a media access control (MAC)technique implemented in the communication system illustrated in FIG. 1.In addition, for example, the transmitting and receiving branches 20-1,20-2, . . . , and 20-N perform a process corresponding to a PHY layer.As described below, frames having different lengths are transmitted froma higher layer. At that time, the lengths of the frames finallytransmitted from the PHY layer are made the same. Note that such controlof the lengths of the frames can be performed by either the dataprocessing unit 25 or the transmitting and receiving branches 20-1,20-2, . . . , and 20-N.

Note that each of the communication stations STA1, STA2, and STA3serving as client devices has a plurality of antennas and performs spacedivision multiple access using an adaptive array antenna. At that time,each of the communication stations STA1, STA2, and STA3 performs userseparation in only reception and does not perform user separation intransmission, that is, transmission frame multiplexing. Accordingly,each of the communication stations STA1, STA2, and STA3 need not have asmany antennas as the access point.

In addition, FIG. 3 illustrates an example of the configuration of acommunication apparatus that complies with an existing standard, such asIEEE 802.11a, and that does not use the space division multiple accessscheme. Among the client devices under the control of the communicationstation STA0 serving as an access point in the communication systemillustrated in FIG. 1, a client device that has the configurationillustrated in FIG. 3 and that performs communication in accordance withonly an existing standard is present.

The communication apparatus illustrated in the drawing includes atransmitting and receiving branch 30 having an antenna element 31 and adata processing unit 35 that is connected to the transmitting andreceiving branch 30 and that processes data to be transmitted andreceived data. In addition, in the transmitting and receiving branch 30,the antenna element 31 is connected to a transmission processing unit 33and a reception processing unit 34 via a duplexer 32.

The data processing unit 35 generates data to be transmitted in responseto a transmission request received from a higher-layer application and,thereafter, outputs the data to the transmitting and receiving branch30. The transmission processing unit 33 performs predetermined signalprocessing, such as encoding and modulation, on a digital basebandtransmission signal and, thereafter, performs D/A conversion on thedigital baseband transmission signal. Subsequently, the data processingunit 35 upconverts the digital baseband transmission signal into an RFsignal. Thus, the power of the signal is amplified. Subsequently, such atransmission RF signal is supplied to the antenna element 31 via theduplexer 32. Thereafter, the transmission RF signal is emitted into theair.

In contrast, upon receiving the RF reception signal from the antennaelement 31 via the duplexer 32, the reception processing unit 34low-noise amplifies the RF reception signal. Thereafter, the receptionprocessing unit 34 down-converts the RF reception signals into an analogbaseband signal and D/A-converts the analog baseband signal.Furthermore, the reception processing unit 34 performs predeterminedsignal processing, such as predetermined decoding and demodulation, onthe analog baseband signal. The data processing unit 35 reconstructs theoriginal transmission data from the digital reception signal input fromthe reception processing unit 34 and delivers the original transmissiondata to a higher-layer application.

In the communication system illustrated in FIG. 1, the communicationstation STA0 serving as an access point multiplies a plurality ofantenna elements 21-1, 21-2, . . . , and 21-N by the weights of anadaptive array antenna. Thus, the communication station STA0 causes theantenna elements 21-1, 21-2, . . . , and 21-N to function as an adaptivearray antenna. In this way, the directivities for the communicationstations STA1 to STA3 can be formed. As a result, the wireless resourceson the spatial axis can be separated for the users, and a plurality offrames destined for the communication stations STA1 to STA3 can bemultiplexed and transmitted at the same time. In addition, byfunctioning as an adaptive array antenna, the communication station STA0can separate the frames transmitted from the communication stations STA1to STA3 at the same time for the users and perform a reception process.

At that time, in order for the antenna elements 21-1, 21-2, . . . , and21-N to function as an adaptive array antenna, the weights of theadaptive array antenna need to be learned in advance. For example, thecommunication station STA0 can learn the weights of the adaptive arrayantenna by acquiring a transfer function from training signals receivedfrom the communication stations STA1 to STA3 in a known sequence.Alternatively, the communication station STA0 can directly learn theweights of the adaptive array antenna using training signals receivedfrom a plurality of the communication partners in a known sequence and apredetermined adaptive algorithm, such as an RLS (Recursive LeastSquare) algorithm.

In either of the above-described techniques, the communication stationSTA0 that learns the weights of an adaptive array antenna needs toreceive training signals from the communication stations STA1 to STA3.In addition, in a communication environment in which a communicationapparatus that complies with only an existing standard exists, a normalframe exchange sequence needs to be executed while avoiding collision ofcarriers. Similarly, the training signals need to be transmitted whileavoiding interference with the communication apparatus that complieswith only an existing standard. That is, the communication station STA0needs to learn the weights of the adaptive array antenna whilemaintaining downward compatibility with the existing standard.

FIG. 4 illustrates an example of a communication sequence for learningthe weights of the adaptive array antenna using training signals. In theexample illustrated in the drawing, a communication station thatperforms learning transmits a Training ReQuest (TRQ) frame forrequesting transmission of a training signal. Upon receiving the TRQframe, each of the neighboring stations returns a training frameincluding a known sequence used for the learning process. Note thatalthough the communication station STA4 in FIG. 4 is not illustrated inFIG. 1, the communication station STA4 is a communication station thatcomplies with only an existing standard and that is a hidden terminallocated in at least one of the communication areas of the communicationstations STA0 to STA3.

The communication station STA0 serving as an access point senses aphysical carrier in advance and confirms that a medium is clear.Furthermore, the communication station STA0 performs back-off andtransmits a TRQ frame. At that time, the communication station STA0 hasnot yet learned the weights of the adaptive array antenna (i.e., theantenna elements 21-1, 21-2, . . . , and 21-N have not yet function asthe adaptive array antenna). Accordingly, the communication station STA0omnidirectionally transmits the TRQ frame.

The TRQ frame includes a field that complies with IEEE 802.11, which isan existing standard. The field contains duration information thatrequests the communication apparatus that is not a destination of theTRQ frame (the hidden terminal) to set, in NAV, a counter valuecorresponding to a duration until a signal transmitting and receivingsequence is completed.

Upon receiving a TRQ frame that does not include a destinationindicating the communication station STA4, the communication stationSTA4 that complies with an existing standard sets a NAV counter valueusing the duration information included in the frame. Thus, thecommunication station STA4 does not perform a transmitting operation. Inaddition, according to the layout of the communication stationsillustrated in FIG. 1, the TRQ frame transmitted from the communicationstation STA0 reaches each of the communication stations STA1 to STA3. Inresponse to the TRQ frame received, after a predetermined frame intervalSIFS (Short Inter Frame Space) has elapsed since the TRQ frame includinga destination indicating the communication station was received, each ofthe communication stations STA1 to STA3 returns a training frameincluding a known sequence usable for training of the adaptive arrayantenna.

According to the present embodiment, learning of the weights of theadaptive array antenna is performed while maintaining downwardcompatibility with an existing standard. Therefore, the training framehas a front field and a rear field. The front field complies with IEEE802.11, which is an existing standard. The rear field contains a knownsequence for training and does not have downward compatibility with theexisting standard. In order to stop a transmitting operation untilneighboring stations that comply with the existing standard complete aseries of signal transmitting and receiving operation, spoofingdescribed in the drawing is performed on the front field that complieswith the existing standard so that such misinterpretation that thetraining frame continues until subsequently performed transmission ofACK is completed occurs. Note that the spoofing technique is describedin more detail in, for example, Japanese Unexamined Patent ApplicationPublication No. 2008-252867 whose patent right has already been assignedto the applicant of the present invention.

In addition, in the example illustrated in FIG. 4, the communicationstations STA1 to STA3 transmit the training frames at the same time.

At that time, the training frames may be transmitted in a timemultiplexed manner. However, if the training frames are transmitted in atime multiplexed manner, a period of time needed for transmission of allof the training frames (i.e., a transmission waiting time needed foreach of the neighboring stations) increases with an increase in thenumber of communication stations that send back the training frames(i.e., the number of communication stations to be learned). Accordingly,the throughput of the whole system decreases, and the overhead of thewhole system increases. In addition, the neighboring station that canreceive only a training frame transmitted at a later time on the timeaxis (the hidden terminal) may start a transmitting operation before thetraining frame arrives, since the NAV counter value disappears. Thus,collision of the carries cannot be prevented. For these reasons,according to the present embodiment, the training frames are transmittedat the same time.

In contrast, after transmission of the TRQ frame has been completed, thecommunication station STA0 enters a ready mode until the training framestransmitted from the communication stations STA1 to STA3, which aredestinations of the TRQ frame, are received. When the training frame isreceived, the communication station STA0 has not yet learned for theadaptive array antenna. Accordingly, the communication station STA0needs to receive the plurality of training frames at the same time usingany one of the antenna elements. At that time, if the following threeconditions are satisfied, the communication station STA0 can receive thefront field sections of the training fields transmitted at the same time(the front field sections having backward compatibility with theexisting standard) while preventing collision.

(1) To employ an OFDM modulation technique.(2) To operate the oscillators of the communication stations STA1, STA2,and STA3 so that a frequency error with respect to the oscillator usedin the communication station STA0 is compensated for.(3) To make information items in the fields of the training framestransmitted from the communication stations STA1, STA2, and STA3 thesame.

It is known that the OFDM modulation technique suggested in thecondition (1) has advantages including its robustness to multipathfading. In addition, the condition (2) can be satisfied if thecommunication stations STA1, STA2, and STA3 perform frequency correctionupon receiving a TRQ frame from the communication station STA0. Byperforming frequency correction, it is assured that the delay times whenthe training frames transmitted from the communication stations STA1,STA2, and STA3 at the same time reach the communication station STA0 arewithin a guard interval. Furthermore, as the condition (3) implies, ifthe information items in the fields of the communication stations STA1,STA2, and STA3 are the same, these fields can be handled as a normaldelayed wave. Thus, the training frames can be received at the same timeby using a single antenna element.

In addition, the communication station STA0 receives the rear field ofthe training frame that contains a known sequence for training and thatdoes not have backward compatibility with the existing standard by usingthe antenna elements 21-1, 21-2, . . . , and 21-N. By assigning aparticular code sequence to each of the communication stations STA1,STA2, and STA3 as the known sequence for training in advance, thecommunication station STA0 can spatially separate the sequences.However, if the number of communication stations that perform a spacedivision multiple access through space division is increased, the lengthof the known sequence inevitably increases since the communicationstations need to be distinguished from one another.

Subsequently, the communication station STA0 learns the weights of theadaptive array antenna using the known sequences and a predeterminedadaptive algorithm, such as the RLS algorithm. Thereafter, the antennaelements 21-1, 21-2, . . . , and 21-N of the communication station STA0can function as an adaptive array antenna, and the communication stationSTA0 can perform the space division multiple access.

In contrast, upon receiving the above-described training frame that doesnot include STA4 as the destination, the communication station STA4 thatcomplies with only the existing standard misunderstands that thetraining frame continues until the subsequent ACK frame is transmitteddue to spoofing (as described above). Thus, the communication stationSTA4 does not perform a transmitting operation.

After a predetermined frame interval SIFS has elapsed since thecommunication station STA0 received the training frames sent from thecommunication stations STA1, STA2, and STA3, the communication stationSTA0 transmits data frames (Fragment0-1, Fragment0-2, and Fragment0-3)to the communication stations STA1, STA2, and STA3, respectively. Byusing the above-described learned weights of the adaptive array antenna,the communication station STA0 can transmit a plurality of data framesthrough space division multiple access at the same time.

In contrast, upon receiving the data frames (Fragment0-1, Fragment0-2,and Fragment0-3) destined for the communication stations STA1, STA2, andSTA3, the communication stations STA1, STA2, and STA3 send back ACKframes (ACK1, ACK2, and ACK3), respectively, at the same time after thepredetermined frame interval SIFS has elapsed.

The plurality of antenna elements 21-1, 21-2, . . . , and 21-N of thecommunication station STA0 have already functioned as an adaptiveantenna. Accordingly, the communication station STA0 can spatiallyseparate the plurality of ACK frames (ACK1, ACK2, and ACK3) received atthe same time for each of the users. For example, by storing, in the ACKframes, the addresses of the communication stations STA1, STA2, and STA3as transmitter addresses, the communication station STA0 can identifythe sources of the received ACK frames. In addition, if the knownsequences for training are stored in even the ACK frames, thecommunication station STA0 can change the learned weights of theadaptive array antenna in accordance with the known sequences fortraining stored in the received ACK frames so that the weights canadaptively follow a change in the environment.

Upon receiving the data frame that is not destined for the communicationstation STA4, the communication station STA4 that complies with theexisting standard sets a NAV counter value on the basis of the durationinformation contained in the frame. In this way, the communicationstation STA4 stops a transmitting operation. In addition, upon receivingthe ACK frame that is not destined for the communication station STA4,the communication station STA4 that complies with the existing standardsets a NAV counter value on the basis of the duration informationcontained in the frame. In this way, the communication station STA4stops a transmitting operation.

As can be seen from an example of the communication sequence illustratedin FIG. 4, the communication station STA0 that performs a space divisionmultiple access can appropriately learn the weights of the adaptivearray antenna. Furthermore, after the communication station STA0 haslearned the weights of the adaptive array antenna, the communicationstation STA0 can share wireless resources on a spatial axis with aplurality of users and multiplex a plurality of data frames destined forthe plurality of users. Thereafter, the communication station STA0 cantransmit the data frames. In this way, the throughput can be increasedin the case of one-to-many communication, that is, communication among aplurality of users.

In this case, in general, a wireless LAN employs a packet communicationscheme. The amounts of traffic that the users desire differ from user touser. Therefore, the lengths of packets (frames) differ from each other.For example, in the example of a communication sequence illustrated inFIG. 4, it is desirable that the lengths of a TRQ frame, a trainingframe, and an ACK frame be the same. However, for a plurality of dataframes transmitted from the communication station STA0, the lengths ofthe frames transmitted from the MAC layer to the PHY layer may not bethe same due to a difference in a transmitted amount of data fromdestination to destination.

However, when frames destined for a plurality of users are multiplexedand simultaneously transmitted through the space division multipleaccess scheme and if the total transmission power is abruptly changeddue to a difference among the frame lengths, an unstable AGC operationmay disadvantageously occur at the receiver side due to an abrupt changein the received power (as described above).

In addition, if some of the frames to be multiplexed are terminated andthe other frames continue to be transmitted, the communication bandwidthis not efficiently used. Thus, the effect of the space division multipleaccess is decreased. FIG. 5 illustrates frames A and B that havedifferent lengths and that are multiplexed on the same time axis. As canbe seen in the example illustrated in the drawing, the length of theframe B is shorter than the length of the frame A, and aftertransmission of the frame B has been completed, the communicationbandwidth is wasted.

Therefore, even when the lengths of the frames multiplexed at the sametime differ from one another, all the transmitted frames need to finallyhave the same frame length.

For example, among the spatially multiplexed frames, the frames havingshort lengths may be padded in the PHY layer so that the frame lengthsare made the same. FIG. 6 illustrates the case in which, in the exampleof the communication sequence illustrated in FIG. 4, when data to betransmitted to STA1 to STA3 at the same time are delivered from a higherlayer (e.g., the MAC layer) and if the amount of each of data items tobe transmitted to STA2 and STA3 (DATA2 and DATA3) is smaller than thatof a data item to be transmitted to STA1 (DATA1), the data items to betransmitted to STA2 and STA3 (DATA2 and DATA3) are padded so that eachof the lengths of the DATA2 and DATA3 is the same as the length of thedata item having a longer length (DATA1). Thus, the lengths of theframes finally transmitted from the PHY layer are made the same.

However, the padding operation causes overhead since actual data are notincluded. Therefore, it is desirable that the lengths of frames beadjusted without using a padding operation.

Accordingly, the present inventors have developed a technique foradjusting the lengths of two or more frames having short lengths so thatthe lengths match the length of a frame having a long length byintegrating the frames into a single frame through Aggregation. In IEEE802.11n for high-speed communication, the term “Aggregation” is known asa frame format that reduces overhead by configuring a single physicallayer section from a plurality of frames.

FIG. 7 illustrates the case in which, in the example of thecommunication sequence illustrated in FIG. 6, when data to betransmitted to STA1 to STA3 on the same time are delivered from a higherlayer (e.g., the MAC layer) and if the amount of each of data items tobe transmitted to STA2 and STA3 (DATA2 and DATA3) is smaller than thatof a data item to be transmitted to STA1 (DATA1), the data items to betransmitted to STA2 and STA3 (DATA2 and DATA3) are integrated into asingle frame by Aggregation so that the length of the DATA2 and DATA3 isthe same as the length of the data item having a longer length (DATA1).Thus, the lengths of the frames finally transmitted from the PHY layerare made the same.

Upon receiving an Aggregation frame, each of the communication stationsmay refer to the destination information contained in the top portion ofthe frame and extract the data portion destined for the communicationstation. For example, upon receiving the Aggregation frames includingdata items destined for STA2 and STA3, each of the communicationstations STA2 and STA3 recognizes that the frame includes a data itemdestined for itself using the destination information in the top portionof the frame and determines which data item in the frame is destined foritself using the header information attached to each of the data items.In this way, each of the communication stations STA2 and STA3 retrievesthe desired data item.

The access point STA0 may employ a scheme in which the MIMO antennaweight used when an Aggregated frame is transmitted is determined as thesum of the MIMO antenna weights used when the access point STA0individually transmits a frame to all of the communication stations. Inthe example of the communication sequence illustrated in FIG. 6, sincethe transmission power used for the communication stations is dividedfor the communication stations STA1, STA2, and STA3, each transmissionpower is one third of the total power. In contrast, in the example ofthe communication sequence illustrated in FIG. 7, the data itemsdestined for STA2 and STA3 are subjected to Aggregation, the powerneeded for the two communication stations need not be separated. Thatis, by using Aggregation in a space division multiple access scheme, thetransmission power used for each of the communication stations isreduced to one half the total power. Thus, the quality of communicationcan be increased.

In addition, as illustrated in FIG. 7, after the communication stationsSTA1 to STA3 that receive the Aggregation frames complete reception ofall of the Aggregation frames, the communication stations STA1 to STA3send back ACK frames (ACK1, ACK2, and ACK3) destined for STA0, which isthe source of the Aggregation frames, at the same time. For the returnedACK frames, the antenna elements 21-1, 21-2, . . . , and 21-N of thecommunication station STA0 have already functioned as an adaptiveantenna. Accordingly, the antenna elements 21-1, 21-2, . . . , and 21-Ncan spatially separate the plurality of ACK frames (ACK1, ACK2, andACK3) received at the same time for each of the users (as describedabove).

FIG. 8 illustrates a processing sequence in the form of a flowchart inwhich, in the communication sequence illustrated in FIG. 7, thecommunication apparatus illustrated in FIG. 2 functions as the accesspoint (STA0) and transmits frames destined for a plurality ofcommunication stations at the same time in a multiplexed manner. Theaccess point starts the processing sequence illustrated in FIG. 8 inresponse to, for example, the occurrence of a data transmission requestor a data reception request from a higher-layer application.

The access point examines that the medium is clear by performing aphysical carrier sense in advance and further performs back-off. In thisway, if the access point enters a communicable mode, the access pointtransmits a training request (TRQ) frame to one or more communicationstations (STA1 to STA3) to which the access point wants to transmit datain a multiplexed manner (step S1).

Subsequently, after a predetermined frame interval SIFS (Short InterFrame Space) has elapsed since the TRQ frame was transmitted, the accesspoint waits until it receives training frames sent back from thereceivers of the training request (STA1 to STA3) (step S2).

At that time, if the access point has not received a training frame fromany one of the receivers of the training request (STA1 to STA3) (No instep S3), the processing proceeds to a re-transmitting process of theTRQ frame. However, the details of the frame re-transmitting process arenot provided here.

However, if the access point can receive a training frame from at leastone of the receivers of the training request (STA1 to STA3) (Yes in stepS3), the access point learns the weights of the adaptive array antennausing a known sequence for training included in each of the receivedtraining frames.

Thereafter, the access point determines whether the lengths of themultiplexed frames destined for the communication stations from whichthe access point was able to receive the training frames are the same(step S4).

Here, if the lengths of the frames to be multiplexed are the same (Yesin step S4), the access point directly multiplexes the data framesdestined for the communication stations from which the access point wasable to receive the training frames and transmits the data frames aftera predetermined frame interval SIFS has elapsed since the trainingframes were received. Thereafter, this processing routine is completed.

At that time, by using the learned weights of the adaptive arrayantenna, the access point can transmit the data frames destined for theplurality of client devices at the same time through space divisionmultiplexing. However, the access point does not transmit the dataframes to the client device from which the access point has not receiveda training frame, since learning is not performed for the client deviceand it is uncertain as to whether or not the client device is presentwithin the communication area.

However, if the lengths of the frames to be multiplexed are not the same(No in step S4), the access point determines whether among the pluralityof communication stations that are destinations of the multiplexedframes (STA1 to STA3), a combination appropriate for combining theantenna weights is present (e.g., a combination of communicationstations located in the vicinity is present) (step S5).

At that time, one of techniques for determining whether a combination ofcommunication stations located in the vicinity is present is to comparethe values of the antenna weights of the communication stations with oneanother. That is, the access point can determine whether the values ofthe antenna weights of one of the communication stations are close tothose of another communication station as a predetermined reference. Ifthe values of the antenna weights of the communication stations areclose to each other, the access point can determine that a combinationof the communication stations is appropriate. For example, let STA0 havethree antennas. In addition, a combination of the antenna weights of thecommunication station STA1 indicates that the weight of an antenna 1 islarge, the weight of an antenna 2 is medium, and the weight of anantenna 3 is small. A combination of the antenna weights of thecommunication station STA2 indicates that the weight of the antenna 1 islarge, the weight of the antenna 2 is small, and the weight of theantenna 3 is medium. A combination of the antenna weights of thecommunication station STA3 indicates that the weight of the antenna 1 islarge, the weight of the antenna 2 is small, and the weight of theantenna 2 is medium. Then, since the values of the weights of STA2 areclose to the values of the weights of STA3, it can be determined thatSTA2 is appropriately combined with STA3. In addition, another techniquefor determining whether a combination of communication stations is acombination of communication stations located in the vicinity is todetermine whether the locations of communication stations are in thevicinity on the basis of information provided by GPSs (GlobalPositioning Systems) mounted in the communication stations and determinethat a combination of the communication stations located within adistance that satisfies a predetermined reference is appropriate.

If a combination of the communication stations appropriate for combiningthe antenna weights is found (Yes in step S5), it is determined whetherthe lengths of all of the frames destined for the communication stationsin the combination and multiplexed at the same time can be made the sameby performing Aggregation (step S6).

Thereafter, if the lengths of all of the frames multiplexed at the sametime can be made the same by performing Aggregation (Yes in step S6), anAggregation process is performed on the frames of a plurality of users(step S7). More specifically, the MIMO antenna weights that are the sumsof the MIMO antenna weights used when the access point individuallytransmits a frame to all of the communication stations that are thedestinations of the aggregated frames are employed.

Thereafter, if the length of the Aggregated frame is not completely thesame as the length of another frame that is multiplexed at the sametime, one of the frames is appropriately padded so that the lengths ofall of the frames are made the same (step S8).

However, if a combination of the communication stations appropriate forcombining the antenna weights is not found (No in step S5) or if thelengths of all of the frames cannot be made the same even whenAggregation is performed (No in step S6), Aggregation of the frames formultiple users is stopped, and the lengths of all of the frames are madethe same by using only a padding process (step S8).

Subsequently, after the predetermined frame interval SIFS has elapsedsince training frames were received, the frames subjected to adjustmentof the frame lengths are transmitted in a multiplexed manner.Thereafter, this processing routine is completed.

FIG. 9 illustrates a processing sequence in the form of a flowchart inwhich in the communication sequence illustrated in FIG. 7, thecommunication apparatus illustrated in FIG. 2 operates as any one of theclient devices (STA1 to STA3) and receives the frames multiplexed at thesame time from the access point. At that time, upon receiving a TRQframe from the access point (STA0), each of the client devices (STA1 toSTA3) starts the processing sequence illustrated in FIG. 9. Note that atthat time, each of the client devices corrects a frequency error using,for example, L-LTF of the header portion of the received TRQ frame, andit is assured that clock errors among the client devices are within aguard interval.

After a predetermined frame interval SIFS has elapsed since the clientdevice received the TRQ frame from the access point (Yes in step S11),the client device sends back a training frame to the access point (stepS12).

At that time, the client device corrects a frequency error using thereceived TRQ frame. Accordingly, when a plurality of the client devicesthat send back training frames are present, it is assured that the timesat which the training frames arrive at the access point are within theguard interval. Thus, the access point can receive the plurality oftraining frames at the same time using a single antenna element.

Subsequently, after the predetermined frame interval SIFS has elapsedsince the client device transmitted the TRQ frame (Yes in step S13), theclient device enters a ready mode until a data frame transmitted fromthe access point is received. (step S14).

At that time, if the client device cannot receive a data frame or if theclient device cannot decode a received data frame due to a frame error(No in step S15), the client device determines that reception of a dataframe is failed. Thus, this processing routine is completed.Alternatively, the client device may transmit NACK to the access pointin order to ask the access point to re-transmit the data frame.

However, when the client device can receive a data frame from the accesspoint (Yes in step S15) and if the predetermined frame interval SIFS haselapsed since the data frame was received (Yes in step S16), the clientdevice sends back an ACK frame to the access point (step S17). In thisway, this processing routine is successfully completed.

As can be seen from FIGS. 7 and 8, in the communication system accordingto the present embodiment, even when the lengths of frames for users arenot the same at a point in time when the communication apparatusreceives a transmission request from a higher-layer application, thecommunication apparatus can integrate at least two frames having shortlengths into a frame having a long length through Aggregation andfinally transmit the frames having the same frame length aftermultiplexing the frames at the same time.

The term “Aggregation” refers to a frame format for reducing overhead byconfiguring a single physical-layer data portion from a plurality offrames in IEEE 802.11n regarding high-speed communication. According tothe present embodiment, by integrating frame aggregation into a spacedivision multiple access scheme, overhead can be reduced and a highthroughput can be obtained in one-to-many communication.

In addition, when the communication apparatus multiplexes a plurality offrames having a variable-length format at the same time, thecommunication apparatus adjusts the frame lengths so that the framelengths are the same. In this way, an abrupt change in the totaltransmission power can be prevented. On the receiver side of themultiplexed frames, an unstable AGC operation caused by an abrupt changein the received power can be prevented. Furthermore, since, in terms ofan Aggregation frame, the number of frames to be multiplexed can bereduced, the transmission power used per destination communicationapparatus on the transmitter side can be increased. Therefore, anincrease in the communication quality can be expected.

INDUSTRIAL APPLICABILITY

While the present invention has been described in detail with referenceto particular embodiments, various modifications and alterations of thisinvention will become apparent to those skilled in the art withoutdeparting from the scope and principles of this invention.

While the present specification has been described with reference toembodiments employing a new wireless LAN standard, such as IEEE 802.11acaiming at ultra-high throughput (1 Gbps), the scope of the invention isnot limited thereto. For example, the present invention is similarlyapplicable to another wireless LAN system in which wireless resources ona spatial axis are shared by a plurality of users and a variety ofwireless systems other than a LAN.

That is, it is understood that the embodiments described herein are forillustrative purposes only and the above disclosure is not intended tobe limiting. The scope of the invention should be determined by theappended claims.

REFERENCE SIGNS LIST

-   20-1, 20-2, transmitting and receiving branch-   21-1, 21-2, antenna element-   22-1, 22-2, duplexer-   23-1, 23-2, transmission processing unit-   24-1, 24-2, reception processing unit-   25 data processing unit-   30 transmitting and receiving branch-   31 antenna element-   32 duplexer-   33 transmission processing unit-   34 reception processing unit-   35 data processing unit

1. A communication apparatus, comprising: circuitry configured to:generate a plurality of MAC frames for transmission via a multiplexoperation; integrate at least two MAC frames of the generated pluralityof MAC frames into a single integrated MAC frame as a first PHY frame;pad, by PHY padding, the first PHY frame based on a length of the firstPHY frame that is shorter than a length of a second PHY frame, togenerate a plurality of PHY frames that have a same length; concurrentlytransmit the generated plurality of PHY frames via the multiplexoperation to a plurality of receiving communication apparatuses, whereinthe plurality of PHY frames comprises a training sequence havingbackward compatibility with a conventional standard; and receive, fromthe plurality of receiving communication apparatuses, a plurality of ACKframes in response to the transmitted plurality of PHY frames, whereinthe plurality of ACK frames are transmitted after a first duration fromreception of the plurality of PHY frames.
 2. The communication apparatusaccording to claim 1, wherein the circuitry is further configured toadjust lengths of the plurality of PHY frames to make the lengths same.3. The communication apparatus according to claim 2, wherein thecircuitry is further configured to integrate the at least two MAC frameshaving first lengths into a single MAC frame having a second length, andwherein each of the first lengths is shorter than the second length. 4.The communication apparatus according to claim 3, wherein thecommunication apparatus further comprises a plurality of antennaelements configured to function as an adaptive array antenna byassignment of weights to the plurality of antenna elements.
 5. Thecommunication apparatus according to claim 4, wherein the circuitry isfurther configured to pad, by PHY padding, a MAC frame of the pluralityof MAC frames to have a same length for each of the plurality of PHYframes, based on the integration that fails to make the lengths of theplurality of PHY frames same.
 6. The communication apparatus accordingto claim 1, wherein the plurality of ACK frames is transmittedconcurrently by the plurality of receiving communication apparatuses. 7.A communication method, comprising: in a communication apparatus:generating a plurality of MAC frames for transmission via a multiplexoperation; integrating at least two MAC frames of the generatedplurality of MAC frames into a single integrated MAC frame as a firstPHY frame; padding, by PHY padding, the first PHY frame based on alength of the first PHY frame that is shorter than a length of a secondPHY frame, to generate a plurality of PHY frames having a same length;concurrently transmitting the generated plurality of PHY frames via themultiplex operation to a plurality of receiving communicationapparatuses, wherein the plurality of PHY frames comprises a trainingsequence having backward compatibility with a conventional standard; andreceiving, from the plurality of receiving communication apparatuses, aplurality of ACK frames in response to the transmitted plurality of PHYframes, wherein the plurality of ACK frames are transmitted after afirst duration from reception of the plurality of PHY frames.
 8. Thecommunication method according to claim 7, wherein the plurality of ACKframes is transmitted concurrently by the plurality of receivingcommunication apparatuses.
 9. A communication apparatus, comprising:circuitry configured to receive a plurality of PHY frames that isconcurrently transmitted from an access point apparatus via a multiplexoperation; wherein the plurality of PHY frames comprises a trainingsequence having backward compatibility with a conventional standard;transmit a plurality of ACK frames in response to the reception of theplurality of PHY frames, wherein the plurality of ACK frames aretransmitted after a first duration from the reception of the pluralityof PHY frames, and wherein the plurality of PHY frames are generated bythe access point apparatus, based on integration of at least two MACframes of a plurality of MAC frames into a single integrated MAC frameas a first PHY frame; and pad, by PHY padding, the first PHY frame ofthe plurality of PHY frames based on a length of the first PHY framethat is shorter than a length of a second PHY frame.
 10. Thecommunication apparatus according to claim 9, wherein the access pointapparatus adjusts lengths of the plurality of PHY frames to make thelengths same.
 11. The communication apparatus according to claim 10,wherein the access point apparatus integrates the at least two MACframes having first lengths into a single MAC frame having a secondlength, and wherein each of the first lengths is shorter than the secondlength.
 12. The communication apparatus according to claim 11, furthercomprises a plurality of antenna elements configured to function as anadaptive array antenna by assignment of weights to the plurality ofantenna elements.
 13. The communication apparatus according to claim 12,wherein the access point apparatus pads, by PHY padding, a MAC frame ofthe plurality of MAC frames to have a same length for each of theplurality of PHY frames, based on the integration that fails to make thelengths of the plurality of PHY frames same.
 14. The communicationapparatus according to claim 9, wherein the plurality of ACK frames istransmitted concurrently by the plurality of receiving communicationapparatuses.